Data is transmitted from and received by a computer by way of a communications interface. A communications interface standard defines both the physical connections which are made to a communications interface and the electrical characteristics of the signals which are transmittable and receivable by the communications interface. Two communications standards which are frequently used are the Electronic 1.3 Industries Association (EIA) RS232 and RS422 standards. Since the RS232 and RS422 interface standards define communications interfaces having different electrical requirements, the placement of RS232 and RS422 functionality in the same integrated circuit chip requires that power be supplied to meet the different voltage and current needs of the interface.
One way to provide the positive and negative voltage levels required by the RS232 and RS422 portions of the integrated circuit chip is to generate the various voltage levels from a single supply voltage by using a voltage multiplier. Voltage multipliers typically are used to increase the voltage supplied from a fixed voltage power supply by a predetermined factor. Voltage conversion may be achieved by utilizing a series of capacitors which are selectively and sequentially charged and discharged to higher intermediate voltages until the desired voltage is reached. Typically such voltage multipliers known to the art have used two charge transfer capacitors in a charge pump arrangement in which one charge transfer capacitor is charged to provide double the supply voltage and the second charge transfer capacitor is charged to provide triple the supply voltage.
One disadvantage of such this charge pump voltage multiplying circuit is that although most of the circuit is fabricated on a single integrated circuit chip, the charge transfer capacitors are typically large devices which must externally be connected to the integrated circuit chip. The use of multiple external charge transfer capacitors not only increases the number of discrete components in the system, but also utilizes circuit board space which could be otherwise used. As the component density of the circuit board increases, space on the board becomes expensive and a reduction in the discrete component count becomes desirable. Additionally, where the charge transfer capacitors are included within the integrated circuit, it is likewise disadvantageous to employ multiple charge transfer capacitors.
Additionally, charge pumps typically consume power during the voltage tripling process in a manner that is independent of whether or not a load is being supplied with current by the voltage tripler. Such wasteful power consumption becomes an important consideration when a portable power supply is providing the source voltage.
The present invention reduces the number of external charge transfer capacitors and hence both decreases the number of discrete components and saves circuit board space. Additionally, the present invention also optimizes power consumption by detecting the presence of a load and appropriately adjusting the power consumed.